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T. Vemulkar and R. P. Cowburn
is additive for the stray field of one of the domain wall configurations and subtractive
for the other. This creates a selection mechanism for the bead as it approaches the
junction that is reminiscent of a train switching tracks. The bead is driven to the path
with the domain wall that has had its field enhanced at the junction as determined by
the sign of the out of plane field pulse.
This selection process is simulated in a model track junction of permalloy 100 nm
wide, 60 nm thick, and with a 2 μm outer diameter. An in-plane rotating field of 625
Oe was simulated to drive the domain wall motion, and a ±250 Oe out of plane field
was used to create the asymmetric magnetostatic potential energy surface that the
bead observes as it exits the junction. Experimentally this process was demonstrated
on a series of 2.8 μm beads going through a junction in the fabricated track described
above, where the bead switching to path 2 was looked at under various conditions.
The out-of-plane fields ranged from −150 to +150 Oe. It was found that the bead
does not switch to path 2 if its domain wall stray field is diminished by the applied
field which agrees with the simulated result. When the applied field is additive to the
domain wall stray field in path 2, the bead only switches to path 2 where the applied
filed pulse is above approximately 57 Oe. This minimum magnitude of applied field
necessary for switching paths is interesting. It shows that the magnetostatic potential
energy surface the bead experiences needs to be modified by a significant external
field to drive path switching. It was explained that this is perhaps due to differences in
domain wall depinning in the two paths as the domain wall goes through the junction
requiring some minimum field magnitude to remove the inherent bias to the bead
motion.
This study takes the demonstration one step further by experimentally demonstrating that beads of different sizes can be sorted based on this technique since
the potential energy well of interaction of each bead is modified by its magnetic
moment. Each bead size thus has different threshold out-of-plane field pulses necessary to switch it to path 2 in the system. Populations of 2.8 and 5.8 μm beads were
robustly sorted through a junction using the appropriate out-of-plane fields. If the
beads were functionalized to capture different bioanalytes, for example, this sorting
mechanism would allow for separation of these analytes, an important step towards
multiplexed detection. This study showed a simple but powerful method to transport and sort magnetic beads on a silicon chip. This is an exciting route forward
for lab-on-chip-type applications driven largely by magnetic forces and torques and
potentially getting around the challenges associated with microfluidic systems [149]
(Fig. 15.7).
15.4 Future Perspectives
The fundamental challenge for materials developed for clinical techniques is in
achieving the desired materials performance within the required bounds of toxicity that are defined by any in vivo application. Iron oxide-based nanoparticles
still form the mainstay of magnetic nanoparticles relevant for this field particularly
T. Vemulkar and R. P. Cowburn
is additive for the stray field of one of the domain wall configurations and subtractive
for the other. This creates a selection mechanism for the bead as it approaches the
junction that is reminiscent of a train switching tracks. The bead is driven to the path
with the domain wall that has had its field enhanced at the junction as determined by
the sign of the out of plane field pulse.
This selection process is simulated in a model track junction of permalloy 100 nm
wide, 60 nm thick, and with a 2 μm outer diameter. An in-plane rotating field of 625
Oe was simulated to drive the domain wall motion, and a ±250 Oe out of plane field
was used to create the asymmetric magnetostatic potential energy surface that the
bead observes as it exits the junction. Experimentally this process was demonstrated
on a series of 2.8 μm beads going through a junction in the fabricated track described
above, where the bead switching to path 2 was looked at under various conditions.
The out-of-plane fields ranged from −150 to +150 Oe. It was found that the bead
does not switch to path 2 if its domain wall stray field is diminished by the applied
field which agrees with the simulated result. When the applied field is additive to the
domain wall stray field in path 2, the bead only switches to path 2 where the applied
filed pulse is above approximately 57 Oe. This minimum magnitude of applied field
necessary for switching paths is interesting. It shows that the magnetostatic potential
energy surface the bead experiences needs to be modified by a significant external
field to drive path switching. It was explained that this is perhaps due to differences in
domain wall depinning in the two paths as the domain wall goes through the junction
requiring some minimum field magnitude to remove the inherent bias to the bead
motion.
This study takes the demonstration one step further by experimentally demonstrating that beads of different sizes can be sorted based on this technique since
the potential energy well of interaction of each bead is modified by its magnetic
moment. Each bead size thus has different threshold out-of-plane field pulses necessary to switch it to path 2 in the system. Populations of 2.8 and 5.8 μm beads were
robustly sorted through a junction using the appropriate out-of-plane fields. If the
beads were functionalized to capture different bioanalytes, for example, this sorting
mechanism would allow for separation of these analytes, an important step towards
multiplexed detection. This study showed a simple but powerful method to transport and sort magnetic beads on a silicon chip. This is an exciting route forward
for lab-on-chip-type applications driven largely by magnetic forces and torques and
potentially getting around the challenges associated with microfluidic systems [149]
(Fig. 15.7).
15.4 Future Perspectives
The fundamental challenge for materials developed for clinical techniques is in
achieving the desired materials performance within the required bounds of toxicity that are defined by any in vivo application. Iron oxide-based nanoparticles
still form the mainstay of magnetic nanoparticles relevant for this field particularly